My Dog's Pupils Are of Different Sizes

Mammals (this includes both you and your dog) are all about bilateral symmetry. This includes every aspect of your dog's eyes, and specifically, in this case, his pupils. A particularly interesting aspect of the mammalian oculomotor system is that it responds equally to stimulation. This means that if you shine a light in one of your dog's eyes, both pupils will constrict equally. This is known as the consensual response. So, if you ever notice that your dog's pupils are of unequal size, regardless of the lighting, something is seriously wrong.

What to Look For

What to Do

Now ask yourself the following questions:

Has your dog recently suffered any form of head trauma? Trauma to the head is probably the most common reason for anisocoria, or unequal pupil size. It means that there is something affecting the nerves that travel between the eyes and the brain. If the trauma occurred recently and your dog is showing no other negative or worrisome signs, it is reasonable to wait for up to twenty-four hours to see if the pupils return to normal. If you don't notice any improvement, have your dog seen by a veterinary ophthalmologist.

Do you notice any other symptoms associated with either eye? Other symptoms that occur in just one eye could cause either eye to respond by exhibiting pupillary dilation or constriction. Check closely for discolorations, growths, and unusual sagging or tightening of the structures of, and associated with, both eyes. Other findings, depending on their severity, may dictate your course of action. If there is any doubt about what to do when it comes to your dog's eyes, always opt to have your vet take a look.

Has your dog been scratching or rubbing either one of his eyes? Repeated trauma to one eye could result in either pupillary dilation or constriction, but it's more likely that the reason behind the scratching or rubbing would also be the cause of the change in the size of the pupil. Try using an Elizabethan collar, which you can buy at pet supplies stores, to prevent your dog from continuing to traumatize the eye for the time being. Also try following the techniques for flushing and soothing the eye that are outlined in “How to Flush and Treat Your Dog's Itchy, Irritated Eyes” [not available online]. If that doesn't help within a day or two, call your dog's vet.

Has your dog recently gotten into any plants, chemicals, or medications? It is unlikely that an inhaled or ingested plant, chemical, or medication would cause a unilateral change in one of your dog's eyes, but it could cause a pre-existing condition to become obvious. Alternatively, direct exposure of one eye to a plant, chemical, or medication could easily result in a direct effect on that eye. Try flushing both eyes repeatedly with sterile saline solution until you are sure that anything that might have been trapped in any portion of either eye has probably been dislodged and flooded away.

When to Get to the Vet

If your dog was exposed to a plant, chemical, or medication and you flush his eyes repeatedly, but you don't notice improvement within twenty-four hours, seek the help of a veterinary ophthalmologist.